1. Field of the Invention
The present invention relates to a two-stroke cycle engine of a scavenging type that is used as a drive source in a small sized work machine such as, for example, a brush cutter.
2. Description of Related Art
The two-stroke cycle engine of air scavenging type has hitherto been known, in which during the scavenging stroke air for leading scavenging is supplied into a combustion chamber after it has been temporarily introduced into a front end portion of a scavenging passage. In this respect, see the JP Laid-open Patent Publication No. 2004-360656. According to this known engine, during the idling, in order to secure a stable rotation, an air supply valve body in an air supply passage is closed to permit only the air/fuel mixture from an air/fuel mixture supply passage to flow into the crank chamber so that the air/fuel mixture of a concentration optimum to the idling may be supplied from the scavenging passage into the combustion chamber.
It has, however, been found that in the known engine of the type discussed above, if the throttle valve is fully opened at a stretch from the idling condition to permit the engine to be rapidly accelerated, not only the air/fuel mixture supply passage but also the air supply passage is opened rapidly. At this time, whereas the air/fuel mixture flows into the combustion chamber from the air/fuel mixture supply passage through the crank chamber and the scavenging passage, the air flows into the combustion chamber from the scavenging passage without flowing through the crank chamber. In addition, since the air supply passage is closed and empty during the idling, a substantial amount of air is rapidly introduced from the air supply passage into the combustion chamber through the scavenging passage.
As a result, the substantial amount of air flows into the combustion chamber earlier than the air/fuel mixture and, therefore, the air/fuel mixture is diluted to become instantly excessively thinned. Accordingly, at the time transfer from the idling condition to the rapid acceleration is initiated, the air/fuel mixture of a concentration required to accomplish the rapid acceleration fails to be supplied into the combustion chamber and, hence, it is quite often that the failure to accelerate and/or the failure of the engine to rotate is/are easy to occur.
In view of the above, it may be contemplated to preset the air/fuel mixture required during the idling to be enriched, but the enrichment of the air/fuel mixture leads to the necessity of increasing the idling opening of the throttle valve and the air supply valve body is correspondingly necessarily opened. Then the supply of the substantial amount of air into the combustion chamber results in an instability of combustion, accompanied by the fluctuation of the rotation. Also, where a lift up type starter operating mechanism is employed, the lift amount during the starting is reduced by a quantity corresponding to the increase of the idling opening and, therefore, the air/fuel mixture required during the starting will not be sufficiently enriched, resulting in a reduction in startability.
Where the drive is made with the throttle valve set to a medium opening, since the passage sectional area of the air supply passage downstream of the throttle valve is large relative to an air supply passage hole in the throttle valve, a flow region, in which the air does not flow within the air supply passage, emerges and, therefore, a turbulent flow of the air tends to easily occur immediately after the flow from the air supply passage hole in the throttle valve into the air supply passage and the flow of the air becomes instable because of that turbulent flow, with the engine rotation consequently becoming unstable.